1. Field of the Invention
The present invention relates to a method of preparing metallic nanoparticle dispersions having a high metallic content and to printing and coating fluids made from these dispersions with which highly conductive patterns or coatings can be formed at moderate curing conditions.
2. Description of the Related Art
The interest in printing or coating fluids containing metallic nanoparticles has increased during the last decades due to the unique properties of such metallic nanoparticles, when compared to the bulk properties of a given metal. For example, the melting point of metallic nanoparticles decreases with decreasing particle size, making them of interest for printed electronics, electrochemical, optical, magnetic and biological applications.
The production of stable and concentrated metallic printing or coating fluids, which can be printed for example by inkjet printing or screen printing, or coated at high speed, is of great interest as it enables the preparation of electronic devices at low costs.
Typically metallic nanoparticles are prepared by the polyol synthesis methodology as disclosed in Mat. Chem. Phys. 114, 549-555, by a derivative of the polyol synthesis methodology or by an in-situ reduction of metallic salts in the presence of various reducing agents. Such methods are disclosed in for example US2010143591, US2009142482, US20060264518 and US20080220155, EP2147733, EP2139007, EP803551, EP2012952, EP2030706, EP1683592, EP166617, EP2119747, EP2087490 and EP2010314, WO2008/151066, WO2006/076603, WO2009/152388 and WO2009/157393.
In such a polyol synthesis, so called capping agents are often used to stabilize the metallic precursor or metallic nanoparticles. Such capping agents usually contain functional groups such as thiol (—SH), carboxyl (—COOH), or amine (—NH) groups. U.S. Pat. No. 8,197,717 for example discloses a metallic ink comprising metallic nanoparticles made by the polyol synthesis wherein the nanoparticles are capped by a capping material such as polyvinylpyrrolidone (PVP).
After applying the metallic printing or coating fluids on a substrate, a sintering step, also referred to as curing step, at elevated temperatures is carried out to induce/enhance the conductivity of the applied patterns of layers. The organic components of the metallic printing or coating fluids, for example polymeric dispersants or capping agents, may reduce the sintering efficiency and thus the conductivity of the applied patterns of layers. For this reason, higher sintering temperatures and longer sintering times are often required to decompose the organic components.
Such high sintering temperatures are not compatible with common polymer foils, such as polyethylene terephthalate (PET) or polycarbonate, which have relatively low glass transition temperatures. There is thus an interest in lowering the sintering temperatures needed to obtain conductive layers or patterns.
EP-A 2468827 discloses polymeric dispersants, which have a 95 wt % decomposition at a temperature below 300° C. as measured by Thermal Gravimetric Analysis. By using metallic printing or coating fluids comprising such polymeric dispersants, the sintering temperature and time could be reduced. In EP-A 11194791.7 and EP-A 11194790.9 both filed on 21-12-2011 a so called sintering additive is used in combination with a polymeric dispersant of EP-A 2468827 to further lower the sintering temperature.
EP-A 12170774.9, filed on 05-06-2012, discloses a metallic nanoparticle dispersion comprising a dispersion medium characterized in that the dispersion medium comprises a specific solvent, for example 2-pyrrolidone. When using such a solvent as dispersion medium, no polymeric dispersants are necessary to obtain stable metallic nanoparticle dispersions.
A disadvantage of metallic nanoparticle dispersions is often their limited content of metallic nanoparticles. To prepare a proper coating or printing fluid, it is therefore often necessary to perform a concentration step on such dispersions. Moreover, storing diluted dispersions may often result in stability issues.
US2010/0084599 discloses the preparation of a silver nanoparticle composition containing a short-chain and a long-chain capping agent adsorbed on the silver particles. The capping agents are both anionic polyelectrolytes having a specific Molecular Weight.
U.S. Pat. No. 7,931,941 discloses a process for the synthesis of metallic nanoparticles by chemical reduction of metal salts in the presence of carboxylic acids including from 3 to 7 carbons capable of binding to the metal particles surfaces and stabilizing them against agglomeration. The synthesis is carried out in water.
US2010/0090179 discloses a process for producing carboxylic acid-stabilized silver nanoparticles wherein silver salt particles are reduced in a solution containing the carboxylic acids.